Apparatus and method for attenuating acoustic waves in propagating within a pipe wall

ABSTRACT

A method and apparatus for damping an ultrasonic signal propagating in the wall of a pipe, the apparatus including at least one damping structure for securing at least one sensor to the wall of the pipe, wherein the at least one sensor includes a transmitter component and a receiver component for transmitting and receiving an ultrasonic signal, wherein the at least one damping structure is associated with the outer wall of the pipe for damping the ultrasonic signal propagating within the wall of the pipe and a processor that defines a convective ridge in the k-ω plane in response to the ultrasonic signals, and determines the slope of at least a portion of the convective ridge to determine the flow velocity of the fluid.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

The present application relates to U.S. patent application Ser. No.10/756,977, filed Jan. 13, 2004, and U.S. Provisional Patent ApplicationNo. 60/833,651, filed Jul. 27, 2006, U.S. Provisional Patent ApplicationNo. 60/856,243, filed Nov. 1, 2006, and U.S. Provisional 60/856,987,filed Nov. 6, 2006, which are incorporated by reference herein in theirentirety.

TECHNICAL FIELD

This invention relates to a method and apparatus for attenuatingacoustic waves (or ring around acoustics) propagating through the wallsof a pipe for a clamp-on ultrasonic flow meter.

BACKGROUND

Most ultrasonic flow measurements seek to leverage information containedin fluid borne disturbances of a specific temporal frequency. Thespecific frequency often results from natural frequencies of the driveelectronics, the transducer, or the resonant transmission characteristicof the pipe wall.

Referring to FIG. 8, one of the primary challenges associated withclamp-on ultrasonic flow metering is the interference between thestructural borne ultrasonic signal component 100 and the desired fluidborne ultrasonic signal component 102. The structural borne component100 of the ultrasonic signal is often of the same or similar frequencyand essentially masks the fluid borne component 102 of the ultrasonicsignal.

Standard pipes are fairly effective waveguides for structural borneacoustics components 100. The ultrasonic pulse propagates along the wallof a pipe 104 with very little damping and rings around thecircumference numerous times until the inherent damping in the pipe andthe propagation of energy axially away from the initial excitationeventually dissipates the structural borne ultrasonic waves.

SUMMARY OF THE INVENTION

An apparatus for damping an ultrasonic signal propagating in the wall ofa pipe is provided, wherein the apparatus includes a structural housingfor securing at least one sensor having a transmitter component and areceiver component for transmitting and receiving an ultrasonic signal,wherein the housing is coupled to the outer wall of the pipe for dampingthe ultrasonic signal propagating within the wall of the pipe.

An apparatus for damping an ultrasonic signal propagating in the wall ofa pipe is provided, wherein the apparatus includes at least one dampingstructure for securing at least one sensor to the wall of the pipe,wherein the at least one sensor includes a transmitter component and areceiver component for transmitting and receiving an ultrasonic signal,wherein the at least one damping structure is associated with the outerwall of the pipe for damping the ultrasonic signal propagating withinthe wall of the pipe. A processor is also provided, wherein theprocessor defines a convective ridge in the k-ω plane in response to theultrasonic signals, and determines a slope of at least a portion of theconvective ridge to determine a flow velocity of a fluid flowing withinthe pipe.

A method for damping an ultrasonic signal propagating within the wall ofa pipe is provided, wherein the method includes introducing anultrasonic signal into a pipe having a fluid flowing within, modifyingthe damping characteristics of the pipe wall by providing multipleimpedance changes in the pipe wall and by providing alternate energydissipation paths for the ultrasonic signals and processing a transittime of the received ultrasonic signals to determine a flow velocity ofthe fluid.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawing wherein like items are numbered alike inthe various Figures:

FIG. 1 is a block diagram of a flow meter having an array of ultrasonicsensor units disposed axially along a pipe for measuring the volumetricflow of the fluid flowing in the pipe, in accordance with the presentinvention.

FIG. 2 is a cross-sectional view of a pipe having a turbulent pipeflowing having coherent structures therein, in accordance with thepresent invention.

FIG. 3 is a block diagram of an alternative embodiment of a sensingdevice of a flow meter embodying the present invention similar to thatshown in FIG. 1.

FIG. 4 is a block diagram of an alternative embodiment of a sensingdevice of a flow meter embodying the present invention similar to thatshown in FIG. 1.

FIG. 5 is a block diagram of an alternative embodiment of a sensingdevice of a flow meter embodying the present invention similar to thatshown in FIG. 1.

FIG. 6 is a block diagram of an alternative embodiment of a sensingdevice of a flow meter embodying the present invention similar to thatshown in FIG. 1.

FIG. 7 is a perspective and cross-sectional view of an embodiment of astructurally significant housing clamped on to a pipe, in accordancewith the present invention.

FIG. 8 is a cross-sectional view of structurally borne and fluid bornecomponents propagating through a pipe wall having an ultrasonic sensorattached thereto.

FIG. 9 is a cross-sectional view of wrapped and unwrapped pipe wallhaving a housing in accordance with the present invention and oneembodiment having no housing.

FIG. 9B is a table illustrating diaphragm diameter and ultrasonicfrequency as a function of wall thickness.

FIG. 10 is cross-sectional view and an expanded view of a structurallysignificant housing in accordance with another embodiment of the presentinvention.

FIG. 11 is cross-sectional view and a perspective view of a structurallysignificant housing in accordance with another embodiment of the presentinvention.

FIG. 12A is an elevational view and a cross-sectional view of a anotherembodiment of the present invention having piezoelectric patches fordamping structural borne ultrasonic signals in accordance with presentinvention.

FIG. 12B is an elevational view and a cross-sectional view of theembodiment of FIG. 12A.

FIG. 12C is an elevational view and a cross-sectional view of theembodiment of FIG. 12A.

FIG. 12D is an elevational view and a cross-sectional view of theembodiment of FIG. 12A.

FIG. 13 is a block diagram of a flow logic used in the apparatus of thepresent invention.

FIG. 14 is a k-ω plot of data processed from an apparatus embodying thepresent invention that illustrates slope of the convective ridge, and aplot of the optimization function of the convective ridge.

FIG. 15 is a block diagram of an apparatus for measuring the vorticalfield or other flow characteristics of a process flow within a pipe, inaccordance with the present invention.

FIG. 16 is a plot of a signal created by a 1 MHz ultrasonic signal atransducer, in accordance with the present invention.

FIG. 17 is a cross-sectional view of structurally borne and fluid bornecomponents propagating through a pipe wall having an ultrasonic sensorattached thereto

FIG. 18 is a plot of a received ultrasonic signal along with an unwanted‘ring-around’ signal, in accordance with the present invention.

FIG. 19 is a cross-sectional view of structurally borne and fluid bornecomponents propagating through a pipe wall having an ultrasonic sensorattached thereto.

FIG. 20 is a plot showing the phase velocity of supportedcircumferential modes with the wall of a pipe, in accordance with thepresent invention.

FIG. 21 is a cross-sectional view of structurally borne and fluid bornecomponents propagating through a pipe wall having an ultrasonic sensorattached thereto and having a pair of block epoxied to the pipe wall toattenuate the ring-around signal, in accordance with the presentinvention.

FIG. 22 is a plot showing the received signal with and without epoxiedring-around blocks.

FIG. 23 is a diagram illustrating the flow of ultrasonic energy injectedinto a pipe without ring-reducing blocks.

FIG. 24 is a diagram illustrating the flow of ultrasonic energy injectedinto a pipe with ring-reducing blocks.

DETAILED DESCRIPTION

The present invention discloses apparatus' and methods for reducing theimpact of structural borne noise, an unintended by-product of launchingthe fluid born ultrasonic interrogation pulse, on the operation ofclamp-on flow ultrasonic flow meters, as described in U.S. patentapplication Ser. No. 10/756,977, filed Jan. 13, 2004, which isincorporated herein by reference.

FIGS. 1 and 2 illustrate an ultrasonic clamp-on flow meter 110, asdescribed in U.S. patent application Ser. No. 10/756,977, wherein theultrasonic flow meter 110 includes an array of ultrasonic sensors 112having a plurality of ultrasonic sensors 114-120 disposed axially alongthe length of the pipe 104. Each ultrasonic sensor 114-120 comprises atransmitter 122 and a receiver 124. The transmitter 122 provides anultrasonic signal to the corresponding receiver 124, wherein theultrasonic signal is orthogonal to the direction of the flow of a fluid126. While this embodiment of the present clamp-on ultrasonic meter 110is described, one will appreciate that the present invention isapplicable to the other embodiments, such as that described and taughtin U.S. patent application Ser. No. 10/756,977, including embodiments innon-orthogonal ultrasonic signals, pitch and catch configurations, pulseecho configurations, and combined transmitter/receiver ultrasonicsensors, as shown in FIGS. 3-6.

For example, while each of the ultrasonic sensor units 114-120 comprisesa pair of ultrasonic sensors (transmitter and receiver) 122, 124 arediametrically-opposed to provide through transmission, the presentinvention contemplates that one of the ultrasonic sensors 122, 124 ofeach sensor unit 114-120 may be offset axially such that the ultrasonicsignal from the transmitter sensor has an axial component in itspropagation direction, as shown in FIG. 3.

As shown in FIG. 4, the present invention also contemplates that thesensor units 114-120 of the sensing device 112 may be configured in apulse/echo configuration. In this embodiment, each sensing unit 114-120comprises one ultrasonic sensor that transmits an ultrasonic signalthrough the pipe wall and fluid substantially orthogonal to thedirection of flow and receives a reflection of the ultrasonic signalreflected back from the wall of the pipe to the ultrasonic sensor.

Referring to FIG. 5, the sensing device 112 may be configured tofunction in a pitch and catch configuration. In this embodiment, eachsensor unit 114-120 comprises a pair of ultrasonic sensors (transmitter,receiver) 122, 124 disposed axially along the pipe 104 disposed on thesame side of the pipe 104 at a predetermined distance apart. Eachtransmitter sensor 122 provides an ultrasonic signal at a predeterminedangle into the flow 126. The ultrasonic signal propagates through thefluid 126 and reflects off of the inner surface of the pipe 104 andreflects the ultrasonic signal back through the fluid 126 to therespective receiver sensor 124.

FIG. 6 shows another pitch and catch configuration for the sensingdevice 112 contemplated by the present invention. This configuration issimilar to that shown in FIG. 5 except that the sensors disposed betweenthe end sensors function as both a transmitter and a receiver. Thispitch and catch configuration reduces the number of sensors needed tooperate.

Referring back to FIG. 1, the signals S₁(t)-S_(N)(t) received from eachultrasonic sensor 114-120 are processed by an ultrasonic signalprocessor 128 and a signal processor 130 (having an array processor 131)for determining the velocity of the fluid flow and/or volumetric flowrate. The signal processor 130 includes at least one of array processinglogic, as will be described in greater detail hereinafter (See FIGS. 13and 14); and cross-correlation processing logic, as also will bedescribed in greater detail hereinafter (FIG. 15).

One should appreciate that the present invention is applicable to atleast all the configurations of an ultrasonic flow meter consideredherein (as well as others not described herein), and will be describedin greater detail hereinafter.

Specifically, the present invention teaches complimentary approaches toattenuating or eliminating the structural borne component 100 of theultrasonic signal. For example, one embodiment comprises a structurallysignificant housing, and a second embodiment including piezoelectricfilms applied to the outer surface of the pipe 104 to damp out thestructural borne ultrasonic vibrations.

The first embodiment, as shown in FIG. 7, involves the use of astructurally significant housing 132 to clamp-on to the outside of theprocess piping 104. The housing 132 is structurally significant in termsof mass and stiffness as compared to the pipe 104 itself and once theclamp-on ultrasonic meter 110 (see FIG. 1) (including the housing 132)is mounted to the pipe 104, the housing 132 and pipe 104 wallessentially form a single structural body at the ultrasonic excitationfrequencies of interest. The idea is to clamp the structurallysignificant housing 132 to the pipe 104 with sufficient force, possiblywith the addition of epoxy, to effectively modify the ultrasonicvibrational characteristics of the pipe 104.

More specifically, the structurally significant housing 132 essentiallymodifies the structural properties of the entire structural path (orsubstantially the entire path) between the transmitting and thereceiving ultrasonic transducers 122, 124. The structurally significanthousing 132 contacts and reinforces all areas of the pipe 104 except forthe immediate area of the transmitting and receiving transducers 122,124. Given that the flexural stiffness of a plate scales with the cubeof the thickness of the plate, doubling the effective wall thicknessincreases the effective flexural stiffness by a factor of 8. Thus, asone rule of thumb, this invention considers doubling the flexuralstiffness by at least 2× as being “significant” and thus a structuralhousing 132 of the same material as the pipe 104 need only result in a˜25% increase in effective pipe 104 wall thickness to be consideredsignificant. Thus, the present invention enhances the relative abilityof the transmitting and receiving sensors 122, 124 to communicatethrough the fluid 126 with respect to the structurally borne fluid path.

In addition to impeding the propagation of the structural wave component100 from the transmitting sensor 122 to the receiving sensor 124, thedesign of the structurally significant housing 132 can be optimized toincrease the transmission of fluid borne ultrasonic wave component 102.Referring to FIG. 9, with the structurally significant housing 132 inplace, the unreinforced section 103 of the pipe 104 wall effectivelyappears as a clamped diaphragm.

Blevins, Formulas for Natural Frequency and Mode Shapes, (which isincorporated herein by reference) provides formulas for the naturalfrequency of a clamp-on diaphragm. For example, for a clamp-on diaphragmhaving a diameter, a, and thickness, h, for a material of modulus, E,Poisson ratio, v, and a mass per unit area, g, the natural frequency maybe given by,

$f_{ij} = {\frac{\lambda_{ij}^{2}}{2\; \pi \; a^{2}}\left\lbrack \frac{{Eh}^{3}}{12\; {g\left( {1 - v^{2}} \right)}} \right\rbrack}$

where f_(ij) is tabulated.

This formulation neglects the real world stiffening effect of thecurvature of the pipe 104 wall in the unreinforced area and thus willlikely under predict the natural frequency for a given geometry.However, recognizing this limitation, initial calculations show that afor pipe 104 wall of ˜0.3 inches, and unreinforced sensor areas ofroughly 0.75 inches in diameters, a flat plate circular disk hasresonant frequencies on the order of 10,000 Hz to 500,000 Hz, which iswithin the range of ultrasonic transducers. Thus, tuning the naturalfrequency of the diaphragm system that is formed using a structurallysignificant housing 132 with the primary transmission frequency of theultrasonic sensors 114-120—created by either driving the transducer at aspecific frequency, or pulsing the transducer, is both practical andfeasible with commonly available ultrasonic transducers and the designproposed herein.

The standard, unreinforced pipe does demonstrate frequency selectivitywith respect to normal incidence ultrasonic waves. The transmission ofnormal incident ultrasonic waves 102 is maximized at frequencies thatcorrespond to the wavelength of compression waves in the pipe 104 wallbeing an integral number of halfwave lengths,

${\lambda = {{\frac{2\; t}{n}\mspace{14mu} {or}\mspace{14mu} f} = {n\; \frac{c}{2\; t}}}},$

Thus, for a 0.3 inch thick steel pipe, maximum transmission occurs at340 KHz, 680 KHz, 1020 KHz, etc.

The effect of the structurally significant housing 132 would bemaximized if the resonant frequency of the diaphragm system designedabove coincided with one of the frequency of maximum transmission.

The design task of aligning the two resonant frequencies becomes one ofselecting the diameter of the “diaphragm” such that the naturalfrequency of the “diaphragm” lines up with the frequency of maximumtransmission. Inspection of the above equations shows that thiscondition is essentially met for “diaphragms” with radii equal to thethickness of the pipe 104 wall.

Thus, under the simplified, but still realistic assumptions discussedherein, one optimal “diaphragm” diameter may be equal to 2 times thethickness of the pipe 104. These values are tabulated in Table 1, shownin FIG. 9B.

Note that as the pipe 104 wall gets thicker, the optimal “diaphragm”diameter increases. Given the size of conventional transducers, thiseffect may be better leveraged for thick wall pipes, such as those usedin high-pressure oil and gas wells.

Referring to FIG. 10, an additional embodiment of a structurallysignificant housing 200 is shown, wherein the presence of thestructurally significant housing 200 provides multiple impedancechanges, alternate energy dissipation paths, and augmented damping toreduce the level of structural borne noise present to interfere with thefluid borne signal required to make a flow measurement. Specifically,the structurally significant housing 200 includes viscoelastic dampingmaterial 202 introduced into slots 204 in the housing 200. Forstructural waves 100 propagating through the housing, the design of theslots 204 provide for shearing of the viscoelastic material 202,effectively augmenting the damping of the structural wave 100.

Referring to FIG. 11, another embodiment of a structurally significanthousing 300 is shown with viscoelastic damping material 202 attachedbetween the housing 300 and structurally significant plates 302. Thestructurally significant housing 300 and the structurally significantplates 302 serve to constrain the viscoelastic material 202 whendeflected, effectively augmenting the damping of the structural wave100.

While the present invention of a structurally significant housing 132,200, 300 attenuates the structural borne ultrasonic signals 100propagating circumferentially around the pipe 104, one should appreciatethat the housing 132, 200, 300 will also attenuate or eliminate axiallypropagating structural borne ultrasonic signals 100. Further, while thehousing 132, 200, 300 is shown as a single housing comprised of twohalves bolted together to retain the ultrasonic sensors 114-120 of thearray of sensors 112, one should appreciate that the present inventioncontemplates that the ultrasonic meter may comprise a plurality ofdiscreet independent structurally significant housings, wherein eachsensor 114-120 of the array 112 may be mounted to the pipe 104 by arespective structurally significant housing 132, 200, 300. It is furthercontemplated that a housing 132, 200, 300 may also include any number ofultrasonic sensors 114-120 less than the total number of the array 112.

Referring to FIGS. 12A, 12B, 12C and 12D, an additional approach ofattenuating or damping the structural borne ultrasonic signal orvibration 100 includes the use of piezo films 304 applied to the outersurface of the pipe 104. Piezo devices 304 bonded to a vibratingstructure and electrically shunted to dissipate charge generated bydeformation are well known to serve as effective dampening devices forstructural vibration. (e.g. Piezo damping of Fan blades). By tuning theelectrical properties of the piezo RLC circuit 306, the circuit 306 canbe optimized to preferentially damp structural vibration of a specificfrequency.

One objective of the current invention is to bond piezoelectricmaterials (e.g. PVDF film) 304 to the pipe 104 wall along the region ofthe wall in which the interfering structural borne ultrasonic vibration100 (see FIG. 8) would travel. The circuitry 306 could be broadband innature or tuned to optimize attenuation of vibrations at specificfrequencies.

Alternatively to the passive electronic system described above, the pvdffilm 304 could also be used in an active circuit to preferentially dampout specific structural vibration. One piezoelectric film 304 is similarto that shown in U.S. patent application Ser. No. 10/712,833, filed onNov. 12, 2003, which is incorporated herein by reference.

In one configuration envisioned, the pvdf system is applied to the pipe104 as a separate sub system of the existing ultrasonic flow meteringsystem. Typical, piezo transducers are used to launch and detectultrasonic signals. The proposed use of piezo dampers constitute aseparate system designed to reduce or eliminate the structure bornecomponent 100 of the ultrasonic signal, unintentionally generated as aby-product of generating the fluid borne component 200, arriving at theultrasonic detector 124 ideally intended to respond to only fluid borneultrasonic devices. An illustration of one embodiment of this concept isshown in FIGS. 12A, 12B, 12C and 12D.

The compressional wavelength in steel at 1 MHz is approximately 0.2inches. Ideally, the spatial extent of the PVDF patches should target anodd integral number of half wavelengths, namely ˜0.1, 0.3, 0.5 inchesetc.

Referring back to FIG. 1, the flow logic in the processor 130 maydetermine the velocity of each sensor in the array of sensors 114-120using one or both of the following techniques to determine theconvection velocity of vortical disturbances within the process flow 126or other characteristics of the process flow 126 that moves/convectswith the process flow 126 by 1) Characterizing the convective ridge ofthe vortical disturbances or other characteristics using arrayprocessing techniques that use an array 112 of ultrasonic sensors114-120 and/or 2) cross-correlating unsteady variations in theultrasonic signals using ultrasonic sensors 114-120. It should beappreciated that while the sensors 114-120 have been shown anddescribed, the present invention is not limited in this regard and thenumber of sensors can vary. For example, any number of sensors may beused, such as 2 to 16 sensors, without departing from the scope of theinvention.

Referring to FIG. 13, a block diagram illustrating the flow logic 308 inthe processor 130 of FIG. 1 is shown and is used to characterize theconvective ridge of the unsteady variations of the ultrasonic signalsand determine the flow rates. As shown in FIG. 13, the flow logic 308includes a data acquisition unit 310 (e.g., A/D converter) that convertsthe analog signals T₁(t) . . . T_(N)(t) to respective digital signalsand provides the digital signals T₁(t) . . . T_(N)(t) to FFT logic 312.The FFT logic 312 calculates the Fourier transform of the digitizedtime-based input signals T₁(t) . . . T_(N)(t) and provides complexfrequency domain (or frequency based) signals T₁(ω),T₂(ω),T₃(ω), . . .T_(N)(ω) indicative of the frequency content of the input signals. Itshould be appreciated that instead of FFT's, any other technique forobtaining the frequency domain characteristics of the signalsT₁(t)-T_(N)(t), may be used. For example, the cross-spectral density andthe power spectral density may be used to form a frequency domaintransfer functions (or frequency response or ratios) discussedhereinafter.

One technique of determining the convection velocity of the coherentstructures (e.g., turbulent eddies) 314 within the flow 126 is bycharacterizing a convective ridge of the resulting unsteady variationsusing an array 112 of sensors 114-120 or other beam forming techniques,similar to that described in U.S. patent application Ser. No.09/729,994, filed Dec. 4, 2000, now U.S. Pat. No. 6,609,069, which isincorporated herein by reference in its entirety.

A data accumulator 316 accumulates the frequency signals T₁(ω)-T_(N)(ω)over a sampling interval, and provides the data to an array processor318, which performs a spatial-temporal (two-dimensional) transform ofthe sensor data, from the x-t domain to the k-ω domain, and thencalculates the power in the k-ω plane, as represented by a k-ω plot.

The array processor 318 may use standard so-called beam forming, arrayprocessing, or adaptive array-processing algorithms, i.e. algorithms forprocessing the sensor signals using various delays and weighing tocreate suitable phase relationships between the signals provided by thedifferent sensors, thereby creating phased antenna array functionality.In other words, the beam forming or array processing algorithmstransform the time domain signals from the sensor array 112 into theirspatial and temporal frequency components, i.e. into a set of wavenumbers given by k=2π/λ, where λ is the wavelength of a spectralcomponent, and corresponding angular frequencies given by ω=2πv.

It should be appreciated that the prior art teaches many algorithms ofuse in spatially and temporally decomposing a signal from a phased arrayof sensors, and the present invention is not restricted to anyparticular algorithm. One particular adaptive array processing algorithmis the Capon method/algorithm. While the Capon method is described asone method, the present invention contemplates the use, or combined use,of other adaptive array processing algorithms, such as MUSIC algorithm.The present invention also recognizes that such techniques can be usedto determine flow rate, i.e. that the signals caused by a stochasticparameter convecting with a flow 126 are time stationary and may have acoherence length long enough so that it is practical to locate sensors114-120 apart from each other and yet still be within the coherencelength.

Convective characteristics or parameters have a dispersion relationshipthat can be approximated by the straight-line equation,

k=ω/u,

where u is the convection velocity (flow velocity). Referring to FIG.14, a k-ω plot is a plot of k-ω pairs obtained from a spectral analysisof sensor samples associated with convective parameters that areportrayed so that the energy of the disturbance spectrally correspondsto pairings that might be described as a substantially straight ridge,wherein the ridge, in turbulent boundary layer theory, is called aconvective ridge.

To calculate the power in the k-ω plane, as represented by a k-ω plot(see FIG. 14) of either of the signals, the array processor 318determines the wavelength and so the (spatial) wavenumber k, and alsothe (temporal) frequency and so the angular frequency ω, of various ofthe spectral components of the stochastic parameter. There are numerousalgorithms available in the public domain to perform thespatial/temporal decomposition of arrays of sensors 114-120.

The present embodiment may use temporal and spatial filtering toprecondition the signals to effectively filter out the common modecharacteristics and other long wavelength (compared to the sensorspacing) characteristics in the pipe 104 by differencing adjacentsensors 114-120 and retaining a substantial portion of the stochasticparameter associated with the flow field and any other short wavelength(compared to the sensor spacing) low frequency stochastic parameters.

In the case of suitable coherent structures 314 being present, the powerin the k-ω plane shown in the k-ω plot of FIG. 14 shows a convectiveridge 320. The convective ridge 320 represents the concentration of astochastic parameter that convects with the flow 126 and is amathematical manifestation of the relationship between the spatialvariations and temporal variations described above. Such a plot willindicate a tendency for k-ω pairs to appear more or less along a line320 with some slope, wherein the slope indicates the flow velocity.

Once the power in the k-ω plane is determined, a convective ridgeidentifier 322 uses one or another feature extraction method todetermine the location and orientation (slope) of any convective ridge320 present in the k-ω plane. In one embodiment, a so-called slantstacking method is used, a method in which the accumulated frequency ofk-ω pairs in the k-ω plot along different rays emanating from the originare compared, each different ray being associated with a different trialconvection velocity (in that the slope of a ray is assumed to be theflow velocity or correlated to the flow velocity in a known way). Theconvective ridge identifier 322 provides information about the differenttrial convection velocities, information referred to generally asconvective ridge information.

An analyzer 324 examines the convective ridge information including theconvective ridge orientation (slope). Assuming the straight-linedispersion relation given by k=ω/u, the analyzer 324 determines the flowvelocity and/or volumetric flow, which are output as parameters 326. Thevolumetric flow is determined by multiplying the cross-sectional area ofthe inside of the pipe 104 with the velocity of the process flow 126.

As previously noted, for turbulent Newtonian fluids, there is typicallynot a significant amount of dispersion over a wide range ofwavelength-to-diameter ratios. As a result, the convective ridge 320 inthe k-ω plot is substantially straight over a wide frequency range and,accordingly, there is a wide frequency range for which the straight-linedispersion relation given by k=ω/u provides accurate flow velocitymeasurements.

For stratified flows, however, some degree of dispersion exists suchthat coherent structures 314 convect at velocities which depend on theirsize. As a result of increasing levels of dispersion, the convectiveridge 320 in the k-ω plot becomes increasingly non-linear.

-   -   2) Cross-correlating unsteady pressure variations using an array        of unsteady pressure sensors.

Referring to FIG. 15, a processor 400 is provided which usescross-correlation of unsteady variations of the ultrasonic signals todetermine the flow rates. The processing unit 400 of FIG. 15 determinesthe convection velocity of the vortical disturbances within the flow 126by cross correlating unsteady ultrasonic variations using an array ofultrasonic sensors 114-120, similar to that shown in U.S. Pat. No.6,889,562, filed Nov. 8, 2001, which is incorporated herein byreference.

Referring to FIG. 15, the processing unit 400 has two measurementregions located a distance ΔX apart along the pipe 104. Each pair ofsensors 114, 116 and 118, 120 of each region act as spatial filters toremove certain acoustic signals from the unsteady pressure signals, andthe distances X₁, X₂ are determined by the desired filteringcharacteristic for each spatial filter, as discussed hereinafter.

In particular, in the processing unit 400, the ultrasonic signal T₁(t)is provided to a positive input of a summer 402 and the ultrasonicsignal T₂(t) is provided to a negative input of the summer 402. Theoutput of the summer 402 is provided to line 404 indicative of thedifference between the two ultrasonic signals T₁, T₂ (e.g.,T₁−T₂=T_(as1)).

The line 404 is fed to a bandpass filter 406, which passes apredetermined passband of frequencies and attenuates frequencies outsidethe passband. In accordance with the present invention, the passband ofthe filter 406 may be set to filter out (or attenuate) the dc portionand the high frequency portion of the input signals and to pass thefrequencies therebetween. Other passbands may be used in otherembodiments, if desired. Bandpass filter 406 provides a filtered signalT_(asf) 1 on a line 408 to Cross-Correlation Logic 410, describedhereinafter.

The ultrasonic signal T₃(t) is provided to a positive input of a summer412 and the ultrasonic signal T₄(t) is provided to a negative input ofthe summer 412. The output of the summer 412 is provided on a line 414indicative of the difference between the two ultrasonic signals T₃, T₄(e.g., T₃−T₄=T_(as2)). The line 414 is fed to a bandpass filter 416,similar to the bandpass filter 406 discussed hereinbefore, which passesfrequencies within the passband and attenuates frequencies outside thepassband. The filter 416 provides a filtered signal T_(asf2) on a line418 to the Cross-Correlation Logic 410. The signs on the summers 402,412 may be swapped if desired, provided the signs of both summers areswapped together. In addition, the ultrasonic signals T₁, T₂, T₃, T₄ maybe scaled prior to presentation to the summers 402, 412.

The Cross-Correlation Logic 410 calculates a known time domaincross-correlation between the signals T_(asf1) and T_(asf2) on the lines408, 418, respectively, and provides an output signal on a line 420indicative of the time delay τ it takes for an vortical flow field 314(or vortex, stochastic, or vortical structure, field, disturbance orperturbation within the flow) to propagate from one sensing region tothe other sensing region. Such vortical flow disturbances, as is known,are coherent dynamic conditions that can occur in the flow whichsubstantially decay (by a predetermined amount) over a predetermineddistance (or coherence length) and convect (or flow) at or near theaverage velocity of the fluid flow. As described above, the vorticalflow field 314 also has a stochastic or vortical pressure disturbanceassociated with it. In general, the vortical flow disturbances 314 aredistributed throughout the flow, particularly in high shear regions,such as boundary layers (e.g., along the inner wall of the tube 104) andare shown herein as discrete vortical flow fields 314. Because thevortical flow fields (and the associated pressure disturbance) convectat or near the mean flow velocity, the propagation time delay τ isrelated to the velocity of the flow by the distance ΔX between themeasurement regions, as discussed hereinafter.

Referring to FIG. 15, a spacing signal ΔX on a line 422 indicative ofthe distance ΔX between the sensing regions is divided by the time delaysignal τ on the line 420 by a divider 424 which provides an outputsignal on the line 426 indicative of the convection velocity U_(c)(t) ofthe saturated vapor/liquid mixture flowing in the pipe 104, which isrelated to (or proportional to or approximately equal to) the average(or mean) flow velocity U_(f)(t) of the flow 126, as defined below:

U _(c)(t)=ΔX/τ∝U _(f)(t)

The present invention uses temporal and spatial filtering toprecondition the ultrasonic signals to effectively filter out theacoustic disturbances P_(acoustic) and other long wavelength (comparedto the sensor spacing) disturbances in the pipe 104 at the two sensingregions and retain a substantial portion of the ultrasonic signalT_(vortical) associated with the vortical flow field 314 and any othershort wavelength (compared to the sensor spacing) low frequency pressuredisturbances T_(other). In accordance with the present invention, if thelow frequency pressure disturbances T_(other) are small, they will notsubstantially impair the measurement accuracy of T_(vortical).

While the cross-correlation was shown using four sensors, whereby twosensors were summed together to form a sensing region, the inventioncontemplates that each sensing region may only be comprised of one (ormore) sensors disposed at an axial location along the pipe 104.

As mentioned hereinbefore, the present invention contemplates that thehousing and blocks for attenuating the structural ultrasonic signals maybe used with any configuration of ultrasonic sensors 114-120.Specifically any of the three classes of flow meters that utilizeultrasonic transducers, which include transit time ultrasonic flowmeters (TTUF), doppler ultrasonic flow meters (DUF), and crosscorrelation ultrasonic flow meters (CCUF).

CCUF's measure the time required for ultrasonic beams to transit acrossa flow path at two, axially displaced locations along a pipe 104. Withinthis measurement principle, variations in transit time are assumed tocorrelate with properties that convect with the flow 126, such asvortical structure, inhomogenities in flow composition, temperaturevariations to name a few.

CCUF's utilize high frequency acoustic signals, i.e. ultrasonics, tomeasure much lower frequencies, time varying properties of structures inthe flow 126. Like all other cross correlation based flow meters, thephysical disturbances which cause the transit time variations shouldretain some level of coherence over the distance between the twosensors.

Cross correlation ultrasonic flow meters have been around since theearly 1960's. CCUF's are typically much more robust to variations influid composition than the other ultrasonic-based flow measurementapproaches such as transit time and Doppler based methods.

Although CCFU's are operationally more robust than other ultrasonicinterpretation techniques, they suffer from drawbacks attributed to mostcross correlation flow meters, i.e., they are have slow update rates andrelatively inaccurate.

Transit time, defined as the time required for an ultrasonic beam topropagate a given distance, can be measured using a radially alignedultrasonic transmitter and receiver. For a homogenous fluid with a notransverse velocity components flowing in an infinitely rigid tube, thetransit time may be given by the following relation:

t=D/A _(mix)

where t is the transit time, D is the diameter of the pipe 104, andA_(mix) is the speed of sound propagating through the fluid 126.

In such a flow, variation in transit time is analogous to a variation insound speed of the fluid. In real fluids however, there are manymechanisms, which could cause small variations in transit time whichremain spatially coherent for several pipe diameters. For single phaseflows, variations in the transverse velocity component will causevariations in transit time. Variations in the thermophysical propertiesof a fluid such as temperature or composition will also causevariations. Many of these effects convect with the flow. Thus, influenceof transverse velocity of the fluid associated with coherent vorticalstructures 314 on the transit time enables transit time basedmeasurements to be suitable for cross correlation flow measurement forflows with uniform composition properties. The combination ofsensitivity to velocity field perturbation and to composition changesmake transit time measurement well suited for both single and multiphaseapplications.

Despite CCUF's functioning over a wide range of flow composition,standard transit time ultrasonic flow meters (TTUF) are more widelyused. TTUF's tend to require relatively well behaved fluids (i.e. singlephase fluids) and well-defined coupling between the transducer and thefluid itself. TTUF's rely on transmitting and receive ultrasonic signalsthat have some component of their propagation in line with the flow.While this requirement does not pose a significant issue for in-line,wetted transducer TTUF's, it does pose a challenge for clamp-on devicesby introducing the ratio of sound speed in the pipe to the fluid as animportant operating parameter. The influence of this parameter leads toreliability and accuracy problems with clamp-on TTUF's.

CCFU's, utilize ultrasonic transducers to launch and detect ultrasonicwaves propagating normal to the flow path. Refraction of ultrasonicwaves at the pipe/fluid interface is not an issue and the ratio betweensound speed of pipe and the fluid does not direct effect operability.

In still another embodiment, each pair of transducers 114-120 comprise asingle transmitter 122 to emit an ultrasonic signal through the flow 126and a receiver, 124 which receives the respective signal for processing.The time it takes for the signal to arrive at the receiver transducer124 for each pair is calculated and fed to the SONAR algorithms (in thearray processor 131) where the flow rate is calculated. One embodimentuses a very simplistic signal detection algorithm that looks for a peakin the reading obtained from the receiver 124. This algorithm works wellwhen a good signal-to-noise ratio is observed at the receiver 124,however when bubbles intersect the signal path between the transmitter122 and receiver 124 a significant attenuation can occur, which willseverely degrade the received signal quality. The amount of attenuationwill vary depending on the bubble characteristics such as size anddensity.

Referring to FIG. 17, the transmitting ultrasonic transducer array 122is periodically pulsed to create the ultrasonic signal that transmitsthrough the pipe 104 and fluid. Each transducer will have a fundamentaloscillation frequency, which when pulsed will emit a short ultrasonicburst signal. FIG. 16 shows the signal created by a 1 MHz ultrasonictransducer when pulsed with a 10 nS width pulse created in the flowmeter 110. In typical applications the receiving ultrasonic transducer124, located on the opposite side of a pipe 104, will receive thissignal once it has bisected the pipe 104 however in addition to thisprimary through-transmitted signal other unwanted secondary signals willalso be detected. These secondary signals include portions of theoriginal signal that have been refracted or reflected along a differentpath through the pipe 104 than the preferred direct transmission. Oftenthese secondary signals possess sufficient strength to still reach thereceiver transducer 124 and will interfere with the desired signal.Examples of these secondary signals include the ring-around signals 600that travel within the pipe wall 104, reflected signals that may bounceoff multiple interfaces such as the transducer-pipe interface or thepipe-liquid interface, or as in the case here where an array oftransducers are used, from an adjacent transducer, as shown in FIG. 17.

The dominant secondary signal is the ‘ring-around’ signal 600. This isthe portion of the ultrasonic signal that travels around through thewall of the pipe 104 and can still be detected by the receivingtransducer 124. FIG. 18 shows a diagram of this signal as compared tothe through-transmitted signal. As shown in FIG. 19, ultrasonictransmitting and receiving transmitters 122, 124, respectively, areshown attached to the outer surface of a pipe 104. They are arrangedsuch that the generated ultrasonic signal will be normal to thedirection of the fluid flow and travel through the center 602 of theliquid within the pipe 104. As discussed above, as the ultrasonic signaltravels through the pipe 104, bubbles 604 and other matter within thepipe 104 will scatter and attenuate the signal before it fully traversesthe pipe 104 and is detected by the receiving transducer 124. Alsodepicted is the ‘ring-around’ signal 600. This signal is created throughreflection and diffraction between the transmitting ultrasonictransducer 122, the pipe wall 104 and the material present inside thepipe 104 due to the large impendence mismatch between the variousmaterials. As an example, the impedance of steel is 45 MRayls incontrast to fluid which has an impedance of 1.5 MRayls. In this case,only a small percentage of the ultrasonic signal is actually injectedinto the fluid while the rest is reflected throughout the overallsystem. The majority of this excess energy is present in the pipe 104wall in the form of shear and compressional ultrasonic waves 600. Thesewaves will travel throughout the pipe 104 and will be seen by thereceiving transducer 124 along with any desired signals. Coupled withthe fact that the through-transmitted signal can be significantlyattenuated as it travels through the fluid 126 in the pipe 104, it canbe very difficult to distinguish the wanted signal from all thesecondary signals. FIG. 19 shows an example of a received ultrasonicsignal 602 along with an unwanted ‘ring-around’ signal 600. The arrowindicates the location of the through-transmitted pulse in relation tothe large ‘ring-around’ signal. Contrast this to the clean ultrasonicsignal seen in FIG. 16.

To increase the system robustness of the ultrasonic flow meter 110, theamount of the noise signal may be decreased by mechanically reducing thestrength of the secondary ring-around ultrasonic signals that were ableto reach the detectors.

Signal to Noise

It should be appreciated that the quality of any flow measurement,independent of the technology, is typically dependent upon the signal tonoise ratio (S/N). Noise, in this case, is defined as any portion of themeasured signal that contains no flow information. It is desirable tomaximize the S/N to obtain optimum performance. As mentioned, thedominant noise source for the ultrasonic flow meter 110 was determinedto be ring-around noise. Ring-around noise is defined as the signal seenby the receiving transducer 124 that has not passed through the fluid126, but instead traveled via the pipe 104 wall. This signal contains noflow information and, in certain cases, can corrupt the measurement.FIG. 19 shows both the signal path and ring-around path.

The ultrasonic flow meter 110 measures the modulation of thetime-of-flight (TOF) measurement orthogonal to the flow direction. TheTOF modulation is due to the vortical disturbances in the beam path andthe flow velocity is determined by correlating these coherentmodulations over the length of the sensor array.

Under ideal conditions, the ratio of the signal passing through thefluid 126 to the ring-around noise is high, and/or the differential TOFbetween the signals is large, and a flow measurement can be made. Insituations where the straight through signal is attenuated due toproperties of the fluid 124 (air bubbles, particulates, etc.) the S/Nratio can be substantially reduced and the flow measurement compromised.In cases where the signal and noise temporally overlap, and/or insituations where the ring-around signal is greater than the straightthrough signal, advanced signal processing algorithms need to beemployed to detect the signal. In order to reduce the burden placed onthe detection algorithm to detect small signals in the presence of alarge ring-around signal, methods of reducing the amplitude of thering-around noise were investigated.

The properties of the ring-around energy differ depending upon the wallthickness of the pipe 104, transducer frequency, pipe surface quality,and transducer size. Generally speaking, higher levels of ring-aroundare seen at smaller pipe diameters (i.e. 2 inch) for a given transducerexcitation frequency due to the tighter curvature of the wall.Ring-around signals can be generated when energy from the transducer iseither directly coupled into the pipe wall and/or be a result ofreflected energy from the inner pipe/liquid interface. This energy canpropagate as a variety of different waves, such as shear, longitudinaland surface waves. FIG. 20 shows the phase velocity of supportedcircumferential modes within the wall of a schedule 40, 2 inch steelpipe. It can be seen that at low excitation frequencies, such as 1 MHz,four modes can be supported in the pipe wall, wherein the number ofmodes capable of being supported increases with increased frequency. Thephase velocity of the lower order modes converges to approximately 3000meters/sec.

One approach to eliminate ring-around involves coupling the energy intoa mechanical structure attached to the pipe 104. Referring to FIG. 21,two steel blocks 500 were machined with a curvature slightly larger thanthe radius of a 2 inch pipe 104. Acoustic coupling gel was appliedbetween the pipe 104 and the curved face of the blocks 500. The blocks500 were then coupled to the pipe 104 which was then filled with waterand the ring-around noise was measured and compared to the straightthrough signal. This was accomplished by first measuring and recordingthe received signal containing both the ring-around noise and thestraight through signal, followed by a measurement with the straightthrough beam blocked. The difference between the measurements representsthe contribution of the ring-around noise. The results of these testsshowed the blocks had little impact on the attenuation of the acousticenergy propagating in the pipe 104 wall.

A second test was conducted where the blocks 500 were epoxied to thepipe 104 wall. Comparison of these measurements showed substantialattenuation of the ring-around energy. FIG. 22 shows the received signalwith and without epoxied ring-around blocks 500. The first arrivalsignal without ring-around blocks occurs at approximately 31 usecs. Thisis consistence with the calculated transit time through steel. Thestraight through signal containing the flow information has a transittime of 41 usec. Ring-around blocks attenuate the ring-around noiseresulting in an improved signal to noise at the receiver 124. It shouldbe appreciated that improvements in S/N of up to 20 dB were realizedwith ring-around blocks.

If should also be appreciated that while the present inventioncontemplates using a block of material 500 (e.g., steel) attached orengaged to the pipe 104 to attenuate acoustic waves propagating throughthe pipe 104 wall, the invention further contemplates that the blocks500 may be comprised of a sheet of material (e.g., steel, tin and lead)that is epoxied or otherwise engaged or attached to the pipe 104 wall.The sheet material may cover a substantial portion of the circumferenceand length of the array of sensors 114-120. The attenuation design maycomprise of a plurality of respective sheets for each ultrasonic sensorpair and disposed on both sides of the pipe 104 between the sensor pair.

As discussed above and as seen in FIG. 23 and FIG. 24, for variousmeasurements made on pipes 104 the transit time of an ultrasonic wave isdetermined and a related pipe parameter is derived (e.g. flow velocity).Often the ultrasonic energy is coupled through a pipe 104 wall and theninto the confined fluid 126. The signal of interest is the signal thatpasses thru the fluid 126 (or other material contained in the pipe 104).Sometimes this signal is difficult to see because some of the ultrasonicenergy is unavoidably coupled into the pipe 104 wall and travels aroundthe circumference of the pipe 104 wall and ends up on top of the desiredsignal. This unwanted signal is sometimes referred to as ring-round.

By attaching blocks 500 with similar impedance to the pipe 104 to thepipe wall the ring-round can be reduced. The blocks 500 reduce thering-round by basically two methods. First, for a wave traveling in thepipe wall, the block 500 because of its thickness, creates a differentimpedance and the energy is reflected. Second, the energy that is notreflected travels out into the block 500 and does not continue aroundthe pipe 104. Note that the blocks 500 should be attached to the pipewith a solid material because a gel or liquid may not couple out theshear wave.

While the invention has been described with reference to an exemplaryembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, may modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment(s) disclosed herein as thebest mode contemplated for carrying out this invention.

1. An apparatus for damping an ultrasonic signal propagating in the wallof a pipe, the apparatus comprising: a structural housing for securingat least one sensor having a transmitter component and a receivercomponent for transmitting and receiving an ultrasonic signal, whereinsaid housing is coupled to the outer wall of the pipe for damping saidultrasonic signals propagating within the wall of the pipe.
 2. Theapparatus of claim 1, wherein said housing is coupled to the wall of thepipe via an adhesive.
 3. The apparatus of claim 1, wherein saidstructural housing includes at least one slot containing a viscoelasticdamping material.
 4. The apparatus of claim 3, wherein said at least oneslot includes a plurality of slots disposed at least partially along acircumference of said structural housing.
 5. The apparatus of claim 3,wherein said at least one slot is configured to provide for shearing ofsaid viscoelastic material and wherein said structural housing isexposed to said ultrasonic signal.
 6. The apparatus of claim 1, whereinsaid structural housing includes at least one structural plate attachedto the pipe wall and separated from the pipe wall by a viscoelasticmaterial.
 7. The apparatus of claim 1, wherein said transmittercomponent is associated with the pipe wall substantially opposite saidreceiver component.
 8. The apparatus of claim 1, wherein said structuralhousing is configured to augment damping in the pipe wall by providingmultiple impedance changes and alternate energy dissipation paths for anultrasonic signal propagating in the pipe wall.
 9. The apparatus ofclaim 1, wherein said processor determines flow rates of a fluid passingwithin the pipe by sampling said ultrasonic signals and identifying andcross-correlating unsteady variations of said ultrasonic signals. 10.The apparatus of claim 1, wherein a processor samples said ultrasonicsignals to identify and determine an orientation of a convective ridgein the k-ω plane.
 11. The apparatus of claim 1, wherein said ultrasonicsignals are indicative of vortical disturbances with a fluid flowing inthe pipe and wherein a processor uses a beam forming algorithm to definea convective ridge in the k-ω plane.
 12. An apparatus for damping anultrasonic signal propagating in the wall of a pipe, the apparatuscomprising: at least one damping structure for securing at least onesensor to the wall of the pipe, wherein the at least one sensor includesa transmitter component and a receiver component for transmitting andreceiving an ultrasonic signal, wherein said at least one dampingstructure is associated with an outer wall of the pipe for damping saidultrasonic signal propagating within the wall of the pipe; and aprocessor that defines a convective ridge in the k-ω plane in responseto said ultrasonic signals, and determines a slope of at least a portionof said convective ridge to determine a flow velocity of a fluid flowingwithin the pipe.
 13. The apparatus of claim 12, wherein said at leastone damping structure is associated with the wall of the pipe via anadhesive.
 14. The apparatus of claim 12, wherein said transmittercomponent and said receiver component are associated with the pipe wallvia said at least one damping structure.
 15. The apparatus of claim 12,wherein said transmitter component is associated with the pipe wall tobe disposed substantially opposite said receiver component.
 16. Theapparatus of claim 12, wherein said at least one damping structure isconfigured to augment damping in the pipe wall by providing multipleimpedance changes and alternate energy dissipation paths for anultrasonic signal propagating in the pipe wall.
 17. The apparatus ofclaim 12, wherein said processor determines a flow rate of a fluidpassing within the pipe by sampling said ultrasonic signals andidentifying and cross-correlating unsteady variations of said ultrasonicsignals.
 18. The apparatus of claim 12, wherein said processor samplessaid ultrasonic signals to identify and determine an orientation of saidconvective ridge in the k-ω plane.
 19. The apparatus of claim 12,wherein said ultrasonic signals are indicative of vortical disturbanceswith said fluid flowing in the pipe and wherein said processor uses abeam forming algorithm to define said convective ridge in the k-ω plane.20. A method for damping an ultrasonic signal propagating within thewall of a pipe, the method comprising: introducing an ultrasonic signalinto a pipe having a fluid flowing within; modifying dampingcharacteristics of the pipe wall by providing multiple impedance changesin the pipe wall and by providing alternate energy dissipation paths forsaid ultrasonic signals; and processing a transit time of receivedultrasonic signals to determine a flow velocity of said fluid.